In the leech we meet with some variations in the arrangement of the nervous system, of a kind analogous to changes subsequently to be spoken of as occurring in higher forms of life. The nervous system becomes more concentrated. There is no longer a ganglion for each segment, but one for every three or four segments of the animal; and the two ventral cords approximate so closely as to be almost fused into one. In the common medicinal leech, for instance, there is a bilobed ganglion (a) above the mouth, which receives fibres from the tactile lips, and also ten distinct filaments from as many pigment-spots (b b) or ocelli, situated round the margin of this upper lip.
Fig. 2.—Nervous System of the Medicinal Leech.
From this bilobed ganglion, which corresponds with the brain proper of higher animals, a cord descends on each side of the œsophagus, and the two unite in a heart-shaped supra-œsophageal ganglion (c), from which afferent nerves are given off to the muscles whose business it is to move its three saw-like jaws, as well as to the muscles of the oral sucker. This lower ganglion in part corresponds with the "medulla oblongata" of vertebrate animals. It is continuous with the double ventral cord, on which twenty equidistant rhomboidal ganglia are developed. Each of these ganglia gives off two nerves
